I've had similar symptoms twice since reporting this, although the circumstances differ.

One is happening right now. I edited a class and hit F5. It did not save, it did not build, and it did not run. I tried to open the Project menu to see what was going on, but none of the menus opened. Eventually, after switching to other applications, I did manage to get them to open, but the File | Save menu item was disabled. I could not save my change, so I hit Alt+F4. Then I heard a beep, and the task bar showed an extra captionless icon. Then many, continuous beeps ensued. Two more captionless icons appeared in the task bar. In Process Explorer, I can see that ec.exe is using about 1,720 GDI Objects and various files are opening and closing in my Temp directory. There is only one thread.

I had to kill ec.exe.